[PASS] Refactor build config, allow implicit unroll pragma (#167)

docs/api/python/build.rst

examples/operator/gemm/cuda_gemm_square.py

examples/operator/rnn/lstm.py

examples/operator/rnn/matexp.py

include/tvm/ir_pass.h

include/tvm/schedule.h

python/tvm/__init__.py

python/tvm/build.py

src/api/api_pass.cc

src/codegen/codegen_cuda.cc

src/codegen/codegen_cuda.h

src/pass/unroll_loop.cc

tests/python/integration/test_gemm.py

tests/python/unittest/test_pass_unroll.py